SAN JOSE — Dr. Ari Kriegsman walked the frost-covered trails of the trash-strewn homeless encampment known as “The Jungle,” a stethoscope draped around his neck.

“Morning, it’s Valley Med,” he said, approaching a tent. “We know it’s super-cold. Is there anybody around who is sick? We’ve got the medical van here.”

Doctors don’t make traditional house calls anymore. But the type of medicine practiced by the Valley Homeless Healthcare Program is hardly commonplace, with nurses and doctors even shouldering medical backpacks to seek out those who have no permanent place to call home. They head into South Bay encampments to treat some of society’s most vulnerable, where something as minor as the flu can become deadly.

Just days after four men — including three who were homeless — died of exposure in Santa Clara County during an unrelenting cold snap, homeless people slowly emerged from makeshift structures to meet with Kriegsman and other outreach workers. They talked about what ailed them, and some promised to visit the nearby mobile unit with two fully equipped examining rooms.

“The doctors are really helpful,” said Chris Salazar, 50, leaning on a cane. “If not for them, I don’t know where I would be.”

The program, which is part of the Santa Clara Valley Health & Hospital System, treats 7,000 homeless each year. In addition to walk-in clinics, the bus-sized medical vehicles can travel to encampments. Because the homeless — many of whom are suffering from mental illness and addiction issues — sometimes won’t come out to a mobile unit, staff members have designed a unique backpack medicine model.

“We go to the people,” said Mercy Egbujor, a nurse practitioner. “There are so many in need. Nobody deserves to live like this. These aren’t places fit for human habitation. I’ve never met somebody who ever said, ‘I love living here.’ It’s always, ‘I don’t have a choice. I don’t have a place to stay.’ “

The program also is a smart use of resources because the homeless can be a drain on social services such as emergency room visits and ambulance calls, said Charles Preston, a psychologist who handed out snacks from his backpack as he walked The Jungle with Kriegsman.

“When we don’t take care of people with health issues, everyone ends up paying for it,” Preston said. “Most of the people in the creeks don’t use a lot of medical services. But when it does happen, it’s really expensive treating them. So it’s better to help them before they get really sick.”

That’s why Meredith Caballero, 64, was at the mobile unit being treated for a persistent cough. While she’s currently living in shelter housing, Caballero said that she has been homeless for 20 years.

“This is wonderful because most of us have nowhere else to go,” she said. “It’s too far to Valley Med for me. But they take care of your health care right here. If they don’t have something, they’ll go find it for you.”

Communities throughout the country have agencies that specialize in street medicine. For instance, Contra Costa and Alameda counties’ Health Care for the Homeless programs have mobile units that provides services at shelters, community centers and meal sites. The San Francisco Community Clinic Consortium has vans that provide 850 medical visits a year in the city.

But the Valley Homeless Healthcare Program faces an especially difficult challenge: Santa Clara County has the nation’s fifth-largest homeless population, with about 7,600 people counted earlier this year. And 74 percent are living somewhere outdoors, such as in filthy encampments often located next to creeks.

Funded through the county and a federal grant, the program started in 2004 and has grown quickly. Its services now include two clinics as well as mobile medical and dental units that travel to different locations on weekdays. The staff of about 35 has six physicians, nurses, mental-health experts and outreach workers.

Dr. Sara Doorley, the medical director, said among the program’s patients are chronically homeless who have an average life expectancy of just 47. In that population, something as relatively minor as a spider bite quickly can become badly infected if they wash it with creek water.

“For most of us, influenza, unless you’re elderly or young, just is not that big a deal,” she added. “But if you’re homeless, it’s very serious. Last year, we had cases of relatively young people dying of the flu. The homeless just don’t recover well from even simple illnesses.”

The backpack medicine component started two years ago because program officials were finding that some homeless didn’t have a level of trust to even walk a few hundred yards to the mobile unit.

“So we put on the backpacks and knock on tents,” Doorley said.

Arminda Perez has a bag filled with medical supplies that she uses to administer flu shots, immunizations for tetanus and diphtheria, oral HIV tests, checking blood pressure and treating infections. Testing for diabetes, she added, is particularly important.

“Foot sores are a problem here, and diabetes can make it worse,” said Perez, a registered nurse. “They even can end up having limbs amputated. But you don’t know if you have diabetes until you get your blood-sugar level tested.”

On a recent morning, Kriegsman and Preston led the outreach effort, making their way through The Jungle, asking if anyone needed a doctor as social workers handed out blankets in the cold. At the mouth of the encampment, a “waiting room” was set up with folding chairs in the dirt lot as Egbujor saw patients inside the mobile unit.

They treat, on average, about 20 patients during their once-a-week visits to The Jungle.

For his part, Kriegsman said this is the best way he can help the most people.

“I’m hoping I can help reaffirm their faith in the medical system,” he said. “Some of these people always were treated so poorly, and they let conditions go for so long. People are getting health care who otherwise wouldn’t receive it, and they’re really grateful.”

The recent deaths due to exposure focused attention on the desperate plight of homelessness. But the reality, Kriegsman said, is that they have patients who die all the time just because it’s so hard living outdoors.

He recalled one man who camped alone that he checked on several times over a few months. When police later found his body, they theorized he might have been dead a week.

Kriegsman said it hit him hard that he might have been about the man’s only contact with the world.

“These people are so unaccounted for,” he said. “They might not be dying because they’re freezing, but because they don’t have medicine or the right treatment. These deaths are so easy to prevent.”